> OK. FOR A FAQ.
> I just KNOW someone can cut this down in length and add clarity, but
> here's my straw man:
>
> ______________
>
> In RS232 ( also ~= V24) asynchronous transmission, end to end
> synchronisation occurs on a character by character basis. When the
> receiver "sees" the start bit it waits 1.5 bit times (until the
> middle of the first data bit), samples the value of the data bit and
> then continues to sample until all data bits have been input. It then
> waits another bit time and samples once more to confirm that a valid
> stop bit is present. If the transmit and receive speeds are
> consistently slightly different the sampling point will "drift"
> further away from the middle of the bit on each subsequent bit. If
> there is too much difference the sampling point will fall outside the
> intended bit and received data will be incorrect. Limiting acceptable
> difference in send and receive data rates occurs when the sampling of
> the stop bit JUST occurs at either the leading or trailing edge of
> the stop bit. This occurs when the error is enough for the sampling
> point to have "drifted" half a bit during transmission. If there are
> S start bits, D data bits, P parity bits and E stop (end ) bits then
> the error involved is about 0.5 / [S+D+ P + (E-0.5) ] x 100%. (The
> bottom line is the distance in bits from the leading edge of the
> start bit to the middle of the last stop bit). For 1 start bit, 8
> data bits, 0 parity bits and 1 stop bit the allowable error is
> 0.5 / (1 + 8 + 0 + 1 - 0.5) x 100% = 0.5/ 9.5 x 100% = 1/19 x 100% =
> 5.26%. Usually said to be 5% error for standard N81 transmission.
>
> Note that this error speed is the maximum net error allowed for BOTH
> transmit and receive clocks. If both transmit and receive clocks are
> 5% maximum error (e.g. typical internal RC oscillator worst case
> spec) the worst possible net error is about 10% and 'there will be
> trouble". Such an arrangement will often work due to error often not
> being worst case and some components of error sources often (but not
> always ) being shared by transmitter and receiver (e.g. both at about
> the same temperature if in the same location or both in human
> accommodation.)
>
> A ceramic resonator has ample accuracy to guarantee the required
> accuracy if used at both ends. A crystal even more so. Use of a
> ceramic resonator or crystal at one end and a good RC oscillator at
> the other will often result in good operation. "Clever" schemes can
> be devised which determine the minimum time between received bit
> edges "on the fly" and use this as 1 bit time and adjust the clock
> speed accordingly. This allows the use of very inaccurate clocks as
> long as the clock can be adjusted by the system. Bit length
> measurement is also the basis for some auto-baud rate detect schemes
> which determine the correct link speed by having the user enter a
> predetermined character at start up.